Polymer films such as polyester films and triacetyl cellulose films are widely used by virtue of their excellent properties such as transparency and flexibility. The polymer films are used as, for example, window films applied to window panes, touch panel films, ITO substrate films, membrane switch films, 3-dimensional decoration films, and optical functional films for use in flat panel displays. Such films are frequently touched by hands, wiped with clothes, or rubbed by pens, making them susceptible to scratches. For this reason, the above polymer film is made into a multilayer film by providing a so-called hard coat layer coated on its surface before the use.
A substance containing multifunctional acrylic monomer or oligomer is frequently used as a material for a hard coat layer due to the easiness in producing the multilayer film. Such substance is hardened by the irradiation of UV rays or electron beams. A heat and moisture hardening type silica compound is also frequently used as the material for the hard coat layer. Such material is hardened by processes of hydrolysis of alkoxysilane, and dehydrating condensation of silanol generated through the hydrolysis. With the use of the above described materials, a hard coat layer having a sufficient hardness can be formed. However, high amounts of organic solvents contained in a coating liquid which is formed into the hard coat layer are detrimental to environment.
For this reason, an emulsion in which acrylic oligomer is dispersed in water is developed as a coating liquid for forming a hard coat layer.
For example, in Japanese Patent Laid-Open Publication No. 56-161475, it is suggested to use a silanol solution as a coating liquid. The silanol solution is prepared by hydrolysis of a silica compound, specifically, multifunctional alkoxysilane, utilizing water and generated alcohol as solvents. After the application of this coating liquid, a coating layer with high hardness is formed by dehydrating condensation of the silanol in the silanol solution.
However, in a case where a coating layer is formed from the emulsion of acrylic oligomer and water, molecules in the coating layer have hydrophilic groups. Accordingly, the coating layer has higher moisture absorption compared to a coating layer made from a coating liquid in which high amounts of multifunctional acrylic monomer or oligomer is dissolved in an organic solvent. As a result, hardness of the coating layer made from the emulsion of acrylic oligomer and water gradually decreases in high humidity environment, and such hard coat layer lacks sufficient hardness. According to the method described in Japanese Patent No. 1390968, during drying and hardening, the coating layer shrinks due to dehydrating condensation of silanol, resulting in cracks on the coating layer. The cracks remain on the produced hard coat layer. The hard coat layer with the cracks lacks durability and has low optical transmittance.
An object of the present invention is to provide a multilayer film having a hard coat layer and a method for producing this multilayer film. This hard coat layer has high surface hardness, and is excellent in optical transmission, scratch resistance and durability even though the hard coat layer is formed with a water-based coating liquid. Drying of the coating layer formed from the water-based coating liquid has little effect on environment.